Mobile elements form an intrinsic part of all eukaryotic genomes studied to date, demonstrating the enormous success of their amplification and impact on the structure and stability of genomes throughout evolution. The activity of the retroelement LINE-1 (L1) accounts for about one third of the human genome mass. Both the presence and activity of L1 and Alu elements generate genomic instability, through insertional disruption or unequal homologous recombination. Alu and L1 elements together cause about 0.2% of new human germ-line diseases, with an estimate of 1 in 20-100 new human births having a new mobile element insertion. Although SINEs depend on L1 for their own mobilization, their mechanism of insertion deviates from the mechanism of L1 insertion. SINE elements have different requirements for mobilization than L1. Therefore, SINE retroposition may impact the genome in some situations where L1 has little effect. Identifying some of the differences between SINE and LINE amplification will lead to a better understanding of their individual contribution to human disease. With this proposal, we strive to contribute to the understanding of the SINE amplification mechanism, specifically focusing on how SINEs co-opt L1 factors and what contributed to their success. Our preliminary data suggest that SINEs not only depend on L1 factors for their amplification, but that their activity has been modulated by changes in the L1 elements throughout evolution. The rodent SINE, BC1, has a strong preference for a mouse-specific L1, suggesting a species-specific recognition mechanism. In addition, we have data that suggest potential Alu-L1 subfamily-specific interactions. We will expand on both observations and use them as tools with the expectation that they will provide insight into how L1 elements influence SINE amplification. Our approaches will include the evaluation of L1 ORF2p human-mouse chimeric constructs using the tissue culture SINE retropositional assay. SINE RNP sub-cellular distribution will be determined and correlated with L1 RNP localization, including the initial attempts to identify interacting cellular factors. The specific goals of this project are: 1) to evaluate the role of LINE-1 component(s) in the mobilization of BC1, 2) to evaluate the BC1 interaction with the mouse L1 factors, and 3) to evaluate the role of L1 components regulating Alu subfamily amplification. The main goal of this proposal is to closely evaluate these SINE-LINE interactions with the expectation that this will define the differences in the amplification mechanisms utilized by mobile elements, and help explain their differing impacts on human disease. Mobile element activity are major sources of genetic instability causing human disease such as cancer. The long-term goal of this project is to gain a fundamental understanding of what regulates the two most active elements (LINEs and SINEs) and their impact on human health.